The present invention relates to an image forming apparatus for outputting images in accordance with digital image signals through an electrophotographic process. Examples of such an apparatus include a digital printer and a digital copying machine or the like.
There are conventional image forming apparatuses for forming visualized images corresponding to digital image signals bearing density information for pixels.
A laser beam printer or the like is widely used, in which a laser beam emitted in accordance with the image signal scans a surface of an image bearing member such an an electrophotographic photosensitive member or the like having been uniformly charged by charging means so that an electrostatic latent image is formed on the surface of the image bearing member, and the electrostatic latent image is developed into a toner image, which in turn is transferred onto a transfer material by transfer means and then is fixed on a transfer material by fixing means. As a developing means for this use, there is a developing device in which a developer carrying member supplied with a bias voltage having at least a DC voltage component, is faced to the image bearing member, by which the electrostatic latent image is developed into a toner image on the surface of the image bearing member.
In an image forming apparatus for visualizing a digital image signal, it would be considered that there are provided a plurality of input paths, and one path is selected therefrom, and the image forming operation is carried out in accordance with the image signal supplied through the selected input path. The digital image signals are classified into the following two groups. The first is bi-level digital image signals for reproducing dots, lines and two dimensional solid images, such as image signals for documents or the like generated by image signal generating means such as a computer. The second is a multi-level digital image signal which alqo bears tone level (density level) in addition to the shape of the image, such as an image signal read from an original or the like by image reading means using CCD or the like.
In the case of the above-described first type signal, the laser beam printer or the like simply turns on and off the laser beam, since what is required is only to reproduce the shape of the image. In this case, that is, the bi-level image signal, it is desirable that the output is such that the size of one pixel is larger than the theoretical size of one pixel by 10-30% approximately, since then the dot and the line corresponding to one or two pixels approximately, are easily recognized by human eyes. Heretofore, it has been proposed that the size of the one pixel is reproduced to be larger by increasing the developing contrast of the developing means. FIG. 4 shows an example of a relationship between the development contrast and the width of a line of one dot width. A printer which visualizes the bi-level image signal requires means for changing the output property so as to change the thickening of the line width. For this purpose the developing bias voltage is adjusted so as to provide a thin line (larger than the theoretical value by 10%), normal line (20%), or thick line (30%).
In the case of the second type signal for visualizing multi-level image signals, a copying machine type laser beam printer is required to reproduce the tone levels in addition to the shape of the image. To accomplish this, as indicated by a line 2 in FIG. 7, it produces an output bearing information indicative of the density of the image which is linear with respect to the signal level supplied for respective pixel. The relationship between the input signal and the output signal when the laser beam emits the quantities of light proportional to the multi-levels for each pixel (without xcex3 conversion), is determined by the developing property of the developing means and the photosensitive property of the image bearing member. The relation without the xcex3 conversion is not rectilinear in the case of the image bearing members and developing means having been used. FIG. 6 shows an example of the relation without the xcex3 conversion. In the prior art, in order to convert this relation to a rectilinear output-density property, the multi-level image signal supplied to the printer is subjected to a xcex3 conversion process so that the laser is actuated such that the quantity of light for each pixel is proportional to the signal level of the multi-level signal after the xcex3 conversion. This means is widely used.
In the printer or the like which visualizes the multi-level image signals, means for changing the output property in accordance with the state of the original, is required. More particularly, the xcex3 conversion process is changed so that only the background fog is reduced when the original is of the nature tending to produce the background fog, and such that the density is increased when the image density of the original is generally low, in order to provide the output property indicated by lines 1 and 3 in FIG. 7.
The following defects are predicted when there are provided two digital image signal input paths (bi-level and multi-level), one path is automatically or manually selected from the paths, and the image is formed in response to the image signals supplied through the selected input path. When a means for changing the developing contrast is used as a means for changing the image output property, the property of the output image of the multi-level image signal does not change as shown in FIG. 7. In addition, in case of the image produced in response to the multi-level signal or the like, the size of one pixel changes with the result of reduction of the tone gradation in the case of halftone dot image. On the other hand, means for changing the xcex3 conversion process is used to change the image output property, an area of a black portion can not be changed in an image formed by a combination of only white and black dots, line and solid images using bi-level image signals, because the xcex3 conversion is for the multi-level signal. These problems are found when an attempt is made to adjust the image quality when an image is outputted after development of an electrostatic latent image formed in accordance with bi-level and multi-level digital image signals. It is considered that this results from a difference in the electrostatic latent images formed by the bi-level and multi-level signals.
Accordingly, it is a principal object of the present invention to provide a solution to the problem arising when one image forming apparatus processes bi-level and multi-level image signals.
It is another object of the present invention to provide an image forming apparatus and method in which sufficient image quality adjustment is enabled for the bi-level and multi-level image signals.
It is a further object of the present invention to provide an image forming apparatus and method in which the operativity is improved in the image quality adjustment at the time of the image formation by the bi-level and multi-level image signals.
According to an aspect of the present invention, there is provided an image forming apparatus for outputting an image through an electrophotographic process in accordance with a digital image signal, comprising: an electrophotographic photosensitive member; optical means for applying light information to the photosensitive member; developing means for forming a toner image on the photosensitive member; an input path for a plurality of digital image signals containing a bi-level digital image signal and a multi-level digital image signal; control means for changing developing condition of the developing means when the image is outputted in accordance with the bi-level digital image signal, and changing an image forming condition which is different from the developing condition when an image is outputted in accordance with a multi-level digital image signal.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.